Boilers using demineralized or evaporated makeup water or pure condensate are known to be prone to caustic attack. High pressure boilers are particularly susceptible to this type of metal corrosion.
The inside surfaces of the boiler are typically protected with magnetite. Hydroxide ion, being the predominant anion in high purity boiler water, can dissolve the magnetite when highly concentrated. Even though high purity water is being used, caustic (NaOH) can nonetheless become highly concentrated, primarily due to the presence of iron oxide deposits on radiant wall tubes. While the bulk water may contain only 5-10 ppm of caustic, it is quite possible to have localized caustic concentrations of up to 100,000 ppm. The iron oxide deposits are generally porous so that the water is drawn into the porous deposit. Due to heat being applied from beneath, steam is generated and passes out of the porous deposit, while fresh water is again drawn into the porous deposit. The result is the noted high concentration of caustic which must be dealt with if the boiler is to be properly protected.
A widely used method for controlling caustic corrosion in boilers using demineralized (high purity) makeup water, particularly in high pressure boilers, is the coordinated phosphate/pH control treatment. This method of treatment is detailed in an article by George Gibson entitled "The Basics of Phosphate-pH Boiler Water Treatment", Power Engineering, Feb., 1978, page 66, which article is incorporated herein by reference to the extent necessary to complete this disclosure. In any event, portions are excerpted below for purposes of explanation.
The coordinated phosphate/pH corrosion control treatment is based on two principles:
First, that sodium phosphates are a pH buffer; and second, that disodium hydrogen phosphate converts potentially corrosive caustic into relatively harmless trisodium phosphate according to equation 1 below: ##STR1##
Accordingly, general corrosion is prevented through the control of boiler water pH. Adherent deposits with concomitant caustic corrosion are prevented by maintaining a disodium phosphate residual in the boiler water to react with caustic according to Equation 1 above.